This application is based upon Japanese Patent Application No. 2001-206891, filed on Jul. 6, 2001.
1. Field of the Invention
The present invention relates to an oil content measuring device for measuring a refrigerating machine oil within a refrigerant used in a refrigerating machine of an air conditioner, and to a refrigeration system using the same.
2. Description of the Related Art
A refrigerating machine oil (i.e., a lubricating oil) for lubricating a refrigerant compressor in a refrigerating machine is important for ensuring the long-lasting durability of a compressor. Therefore, it becomes very important to know the amount of refrigerating machine oil in the refrigerant. More substantially, how much of the refrigerating machine oil is contained in the refrigerant compressor itself.
As a conventional technology for measuring the amount of a refrigerating machine oil, xe2x80x9cMeasurement of Solubility of Refrigerant in Refrigerating Machine Oil by Electrostatic Capacityxe2x80x9d in Transactions of the Japan Society of Refrigerating and Air Conditioning Engineers, vol. 16, No. 3 (1999) has been well known in the art.
Such a conventional measuring method utilizes the difference in electrostatic capacities between the refrigerating machine oil and the refrigerant. As the electrostatic capacity in a dissolved state of the refrigerating machine oil and the refrigerant have a correlation with the solubility of the refrigerant (it can be defined as an oil content, so it will be referred to as an oil content hereinafter), the above method obtains the oil content by measuring the electrostatic capacity.
In the above conventional method, the relation between the oil content and the electrostatic capacity is obtained using temperature as a parameter and is then prepared as known data, followed by performing a measurement of electrostatic capacity by an electrostatic capacity sensor with the known data to obtain an actual oil content. Therefore, it is a cost effective measurement.
However, in the above conventional method, a target of the measurement is refrigerant in a liquid phase state and it only considers the change in electrostatic capacity using temperature as a parameter to obtain an actual oil content. For instance, if such a method is to be applied in a supercritical refrigerating cycle where a refrigerant in a refrigerating machine is substantially in a supercritical state, a vapor phase state, or a gas-liquid double phase state, it is difficult to simply obtain the oil content by the above conventional method.
In other words, in the above supercritical state or the vapor phase state, compared to the liquid phase state, the electrostatic capacity of the refrigerant dramatically changes not only because of the temperature but also because of the pressure, this results in the need for an enormous amount of data that represents the relation between the electrostatic capacity and the oil content, which need to be prepared in advance. This data is difficult to compile and manage.
In view of the above problem, therefore, it is an object of the present invention to provide an oil content measuring device which is capable of easily measuring an oil content in a refrigerant even though the refrigerant is in a supercritical state or in a vapor phase state. Additionally, it is an object to provide a refrigeration system using such an oil content measuring device.
For solving the above problem, the following technical means are adopted in at least one embodiment of the present invention. According to a first aspect of the present invention, there is provided an oil content measuring device including an electrostatic capacity measuring means (200) for measuring an electrostatic capacity (C) of a refrigerant containing a refrigerating machine oil, a density measuring means (300) for measuring a density () of at least one of the refrigerant and the refrigerating machine oil, a computing means (400) for computing correlation characteristics between the electrostatic capacity (C) and an oil content ("khgr") using the density () measured by the density measuring means (300), the oil content ("khgr") representing a ratio of an amount of the refrigerating machine oil to an amount of the refrigerant containing the refrigerating machine oil, and an oil content determining means (500). The oil content determining means (500) for determining oil content ("khgr"s) from an electrostatic capacity (Cs) measured by the electrostatic capacity measuring means (200) using the correlation characteristics obtained by the computing means (400).
When the oil content measuring device is constructed as above, the electrostatic capacity (C) of the refrigerant can be calculated at the temperature and pressure used in the measurement by making use of the density () of the refrigerant or the refrigerating machine oil even though the refrigerant in the refrigerating machine is in a supercritical state or in a vapor phase state. The electrostatic capacity of the refrigerant changes greatly depending on the changes in temperature and pressure, for example, as in the case of a supercritical refrigerating cycle, to determine the correlation characteristics between the electrostatic capacity (c) and the oil content ("khgr") of the refrigerant. Consequently, it becomes possible to easily obtain the oil content ("khgr") of the refrigerant without preparing an enormous amount of given data in advance.
Furthermore, it becomes possible to prevent the compressor (11) of the refrigeration system (10) from early damage before happens by periodically checking the oil content ("khgr") of the refrigerant at the service store or the like.
Furthermore, in a vapor compression refrigerating cycle having a normal vapor phase state and a normal liquid phase state, a refrigerant in a stable liquid phase state is limited on the outflow side of a cooling body (12) having a super-cooling function. In this case, the conventional measurement can be also allowed to obtain the oil content ("khgr") of the refrigerant at such a position. However, the conventional measurement cannot be performed on both the discharge side and the suction side of the compressor (11) where the refrigerant is in a vapor phase state, so that it is hard to perform a proper measurement in the compressor (11) to obtain the oil content ("khgr") of the refrigerant. According to the present invention, on the other hand, it is possible to perform a proper measurement on each of these sides.
According to a second aspect of the present invention, the computing means (400) performs a computation on the correlation characteristics between the electrostatic capacity (C) and the oil content ("khgr") at temperature and pressure conditions at the time of the measurement by linear interpolation using a density (r) of the refrigerant on the basis of a plurality of correlation characteristics between the electrostatic capacity (C) and the oil content ("khgr") previously prepared under a plurality of temperature and pressure conditions.
In this invention, the electrostatic capacity (C) of the refrigerant is linearly approximated based on the density (r) of the refrigerant, so that it becomes possible to precisely determine the correlation characteristics to be required in the measurement from the given correlation characteristics using the density (r) of the refrigerant.
According to a third aspect of the present invention, the computing means (400) performs a computation on the correlation characteristics between the electrostatic capacity (C) and the oil content ("khgr") using an arithmetic expression previously defined such that the electrostatic capacity (C) is determined by the oil content ("khgr"), a density (r) of the refrigerant, and a density (oil) of the refrigerating machine oil.
In this invention, the assumption is made that the refrigerant and the refrigerating machine oil in a capacitor part (209) of the electrostatic capacity measuring means (200) are placed in series. Thus, the correlation characteristics between the electrostatic capacity (C) and the oil content ("khgr") of the refrigerant are obtained using an arithmetic expression including the density (r) of the refrigerant and the density (oil) of the refrigerating machine oil. Consequently, it is possible to perform a computation on the above correlation characteristics using the above densities (r, oil) which can be easily detected even though there is no given data consisting of correlation characteristics.
According to a fourth aspect of the present invention, the density measuring means (300) includes temperature detecting means (301) for detecting a temperature of the refrigerant and a pressure detecting means (302) for detecting a pressure, and the density () is calculated using detected values obtained by the temperature detecting means (301) and the pressure detecting means (302), respectively. Therefore, the density () can be detected without requiring a complicated device and the oil content ("khgr") can be obtained.
According to a fifth aspect of the present invention, the oil content ("khgr") can be determined by replacing the electrostatic capacity (C) with a relative dielectric constant (xcex5) divided by an electrostatic capacity (Co) in a vacuum. Therefore, the electrostatic capacity (C) can be treated as a nondimensional physical value (xcex5), so that it can always be employed without wasting given data relating to the correlation characteristics even when the specifications of the electrostatic capacity measuring means (200) are changed.
According to a sixth aspect of the present invention, each of the electrostatic capacity (C) and the density () is measured at a position in the vicinity of a suction side or a discharge side of a compressor (11) that compresses the refrigerant. Therefore, the oil content ("khgr") in the vicinity of the compressor (11) itself can be known, so that one can make a precise and quick judgment whether the long-lasting durability of the compressor (11) is appropriate.
According to a seventh aspect of the present invention, a water repellent finishing is applied to surfaces of electrodes (207, 208) of the electrostatic capacity measuring means (200) when each of the electrostatic capacity (C) and the density () is measured at a position in the vicinity of the suction side of the compressor (11). On the suction side of the compressor (11) where the refrigerant reaches a low temperature and a low pressure, the viscosity of the refrigerating machine oil increases and thus tends to adhere to the inside of the electrostatic capacity measuring means (200). According to the invention, it becomes possible to prevent the refrigerating machine oil from adhering to the inside of the electrostatic capacity measuring means (200), permitting precise measurement of the oil content ("khgr") and also permitting a reliable circulation of the refrigerating machine oil in the compressor (11).
According to an eighth aspect of the present invention, a refrigeration system has an accumulator (18) for performing gas-liquid separation on the refrigerant, and an oil-returning flow path (19) for supplying a refrigerating machine oil in the separated liquid-phase refrigerant into a compressor (11) for compressing the refrigerant. The oil content measuring device (100) according to any one of the foregoing aspects of the invention is arranged in piping (21) through which the refrigerant flows. The amount of the refrigerating machine oil passing through the oil-returning flow path (19) varies depending on an oil content ("khgr"s) determined by the oil content measuring device (100). Therefore, the required refrigerating machine oil is supplied to the compressor (11) thereby increasing the durability of the compressor (11).
According to a ninth aspect of the present invention, each of electrodes (207, 208) of the electrostatic capacity measuring means (200) is shaped like a needle and is arranged in the piping (21) through which the refrigerant flows. Therefore, any container for housing the electrostatic capacity measuring means (200) can be eliminated, resulting in saved space and a cost reduction. In addition, since each of the electrodes (207, 208) is shaped like a needle, the electrodes are highly flexible. Additionally, arrangement is possible on a portion where the piping (21) is bent, which results in an excellent mounting ability.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.